1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device in which liquid crystals are sealed between a TFT array substrate and a counter substrate formed with a counter electrode.
2. Description of the Background Art
An active matrix liquid crystal display device includes a TFT (Thin Film Transistor) array substrate, a color filter substrate that faces the TFT array substrate, and liquid crystals that are sealed between these substrates. The TFT array substrate is configured such that scanning signal wiring, image signal wiring, a pixel electrode, and a switching element are arranged in a matrix in a display area. Additionally, a mounting area where an IC and the like are mounted, and a frame area where wiring is drawn from a terminal mounted in the mounting area to the respective signal wirings (the scanning signal wirings, and the image signal wirings) in the display area are present around the display area.
On the color filter substrate (counter substrate), a color filter and a counter electrode are formed. The switching element is turned on by a scanning signal, so that the pixel electrode is charged to the potential of the image signal wiring. At the time of switch off, the potential of the pixel electrode vary (decrease) depending on capacitance between scanning signal line and the pixel electrode. Generally, this potential variation of the pixel electrode is called feedthrough (voltage) or a through voltage. After feedthrough, a voltage applied between the pixel electrode and the counter electrode determines luminance in each pixel.
The active matrix liquid crystal display device that adopts a TFT as a switching element sometimes has a problem that display characteristics become nonuniform in a display surface, for example, flickers occur.
The voltage variation due to feedthrough has a distribution in the display surface, thereby causing the nonuniformity of the display characteristics of the display surface. The reason why this voltage variation becomes nonuniform in the display surface is that different scanning signal delay for each pixel electrode is caused by resistance or capacitance that the scanning signal wiring has.
The larger signal delay is, the higher a pixel electrode potential after feedthrough is. Since an AC potential is applied to image signal wiring in order to prevent burn-in, the center potential of the pixel electrode potential after the feedthrough becomes an optimum common potential (hereinafter, described as optimum Vcom). That is, the larger the signal delay is, the larger the optimum Vcom is (e.g., see Japanese Patent Application Laid-Open No. 2001-154222). When the potential of the counter electrode (i.e., the common potential) is deviated from the optimum Vcom, flickers occur.
Furthermore, a routing wiring from a mounting terminal such as an IC to a display area also causes the delay of a scanning signal. A COG (Chip on Glass) system has a structure in which an IC is directly mounted on a substrate, and a routing wiring on a panel is drawn from the mounting terminal of the IC to the display area. A COF (Chip on Film) system has a structure in which an IC is mounted on a film substrate, and a film is mounted on a panel, and a routing wiring on a panel is drawn from a mounting terminal that connects the film and the panel, to the end of a display area.
In a case where the mounting terminal is arranged on the lower side of the display area, the length of a routing wiring connected to a scanning signal wiring on the upper side of the display area is getting longer, and the length of a routing wiring connected to the scanning signal wiring on the lower side of the display area is getting shorter. Generally, the width of the frame area where routing wirings are arranged is required to be narrowed as much as possible. Therefore, it is often difficult to sufficiently decrease difference in resistance of the routing wirings on the upper side and the lower side by changing the wiring widths of the routing wirings. Accordingly, when a scanning signal is input to a scanning signal input part on the end of the display area, signal delay occurs. Therefore, the optimum Vcom has not only a potential distribution on scanning signal wirings, but also different potential distributions for the respective scanning signal wirings.
As described above, it is necessary to perform not only a countermeasure against signal delay in a direction in which the scanning signal wirings extend, but also a countermeasure against signal delay for each scanning signal wiring.
Japanese Patent Application Laid-Open No. 2002-91391 describes a technology of inclining a distribution of counter potentials by applying different potentials to the both ends of the counter electrode, and bringing the potential distribution of counter potentials close to a potential distribution of the optimum Vcom. However, as described above, the potential distributions of the optimum Vcom are different for the respective scanning signal wirings, and therefore only the uniform inclination of the potential distributions as the entire counter electrode is insufficient.